1. Technical Field
The present invention relates to a synthetic quartz glass to be used for optical components for an apparatus employing ultraviolet lights having wavelengths of at most 400 nm as a light source, and a process for producing it. More specifically, the present invention relates to a synthetic quartz glass to be used as optical components (including products and semifinished products) such as a lens (projection type or illumination type), a prism, an etalon, a photomask, a pericle (pericle material, pericle flame or both) and a material for windows, to be used for light within a range of from the ultraviolet region to the vacuum ultraviolet region emitted from a light source such as an excimer laser (XeCl: 308 nm, KrF: 248 nm, ArF: 193 nm), a F2 laser (157 nm), a low pressure mercury lamp (185 nm), a Xe2* excimer lamp (172 nm) or a deuterium lamp (110–400 nm); and a process for producing it.
2. Background Art
A synthetic quartz glass has such characteristics that it is a transparent material within a wavelength range of as wide as from the near infrared region to the ultraviolet region, it has an extremely small thermal expansion coefficient and is excellent in dimensional stability, and it contains substantially no metal impurity and has a high purity. Accordingly, a synthetic quartz glass has been mainly used for optical components of a conventional optical apparatus employing g-line (436 nm) or i-line (365 nm) as a light source.
Along with high-integration of LSI in recent years, techniques to draw finer and thinner lines has been required in an optical lithography technology to draw an integration circuit pattern on a water, and accordingly use of light having a shorter wavelength as an exposure light source has been promoted. For example, for a light source of a stepper for lithography, a KrF excimer laser or an ArF excimer laser became used, instead of the conventional g-line and I-line, and further, a F2 laser becomes used.
Further, the low pressure mercury lamp, the Xe2* excimer lamp and the deuterium lamp are used for 1) an apparatus such as a photochemical CVD, 2) an etching apparatus or an ashing apparatus for silicon wafer, or 3) an ozonizer, and the development thereof has been made for the future application to optical lithography technology. It is necessary to use a synthetic quartz glass also for optical components to be used by irradiation with light having a short wavelength, such as a gas filling tube to be used for the low pressure mercury lamp, the excimer lamp or the deuterium lamp, or an optical apparatus employing the above-mentioned short wavelength light source.
For the synthetic quartz glass to be used for such optical components, not only optical transmittance of light having a wavelength of from the ultraviolet region to the vacuum ultraviolet region, is required, but also it is required that the transmittance does not decrease due to irradiation with ultraviolet lights (hereinafter referred to simply as durability to ultraviolet light). Further, the optical components to be used by irradiation with light from e.g. the ArF excimer laser, the F2 laser, the low pressure mercury lamp, the Xe2* excimer lamp or the deuterium lamp, are required to be excellent in optical transmittance of light having a wavelength in the vacuum ultraviolet region of at most 200 nm (hereinafter referred to simply as vacuum ultraviolet lights optical transmittance). Further, optical components to be used for light having a wavelength of at most 200 nm, are required to have a smaller refractive index variation range (Δn) as compared with a conventional one (hereinafter referred to as uniformity).
If the conventional synthetic quartz glass is irradiated with light having a high energy emitted from a light source such as the KrF excimer laser or the ArF excimer laser, a new absorption band will be formed on the ultraviolet region, and it has problems as an optical component to organize an optical system employing ultraviolet lays as a light source. Namely, when irradiated with ultraviolet lights for a long period of time, an absorption band of abbreviation 215 nm which is so-called E′ center (≡Si.) and an absorption band of abbreviation 260 nm which is called NBOHC (non-bridging oxygen hole center: ≡Si—O.) will be formed.
The causes of such absorption band formation can be roughly classified into two groups. One cause is a structural defect of the synthetic quartz glass, i.e. an oxygen deficient defect such as ≡Si—Si≡ and ≡Si—H, or an oxidation type defect such as ≡Si—O—O—Si≡, and the other cause is an unstable structure in the synthetic quartz glass, i.e. a three-membered cyclic structure or a four-membered cyclic structure. It is considered that such defects are cut by irradiation with ultraviolet lights, as shown in the following formulae (1) to (4), to form paramagnetic defects (E′ center and NBOHC), and the paramagnetic defects will cause decrease in transmittance, decrease in durability to ultraviolet light, increase in absolute refractive index, variation in refractive index distribution, and fluorescence:≡Si—Si≡+hν→2≡Si.  (1)≡Si—H+hν→≡Si.+H−  (2)≡Si—O—O—Si≡+hν→≡Si—O.  (3)≡Si—O—Si≡+hν→≡Si−+≡Si—O.  (4)
Various methods have been studied to overcome these problems, and it has been known to have hydrogen molecules contained in the synthetic quartz glass in some way. For example, JP-A-3-88742 discloses a method to suppress the decrease in transmittance due to irradiation with ultraviolet lights, by having hydrogen molecules and OH groups contained in the synthetic quartz glass, in an amount of at least 5×1016 molecules/cm3 and in an amount of at least 100 ppm, respectively.
However, the OH groups in the synthetic quartz glass may cause a problem since the reaction of the following formula (5) will proceed due to irradiation with ultraviolet lights to form NBOHC, whereby 260 nm absorption and 650 nm fluorescence will be formed:≡Si—OH+hν→≡Si—O—(NBOHC)+H.  (5)
Even if the hydrogen molecules are contained, the reaction of the formula (5) can not completely be prevented, and particularly when the OH group concentration is high, the 650 nm fluorescence will be more intense, such being problematic. Further, if the OH group concentration is high, the transmittance of light within a range of from 150 to 180 nm will decrease, such being problematic when the synthetic quartz glass is used for an apparatus employing e.g. the low pressure mercury lamp, the Xe2* excimer lamp or the F2 laser as the light source.
To overcome such problems, JP-A-6-227827 discloses a synthetic quartz glass having an OH group concentration of at most 10 ppm and a halogen concentration of at least 400 ppm and containing hydrogen molecules. With the synthetic quartz glass, excellent durability to ultraviolet light can be obtained since the OH group concentration is low, and further a high transmittance can be obtained at a wavelength of from 150 to 180 nm.
Said JP-A-6-227827 proposes a production process comprising (1) a step of subjecting a glass forming material into flame hydrolysis to form a porous quartz glass body, (2) a step of heating the porous quartz glass body under a halogen-containing atmosphere at a temperature of from 800 to 1,250° C. for dehydration treatment, (3) a step of raising the temperature of the porous quartz glass body having dehydration treatment applied thereto, to the transparent vitrification temperature for transparent vitrification, and (4) a step of subjecting the transparent-vitrified synthetic quartz glass to a heat treatment under a hydrogen-containing atmosphere at a temperature of from 500 to 1,100° C. to have the synthetic quartz glass contain hydrogen.
Further, since the synthetic quartz glass is held in an atmosphere containing hydrogen at a high temperature, the oxygen deficient defects of ≡Si—Si≡ and ≡Si—H are likely to be formed, JP-A-8-75901 proposes a production process which comprises forming a transparent-vitrified fluorine-containing quartz glass in substantially the same manner as disclosed in JP-A-6-227827, and then having said quartz glass contain hydrogen in an atmosphere containing hydrogen at a temperature of at most 500° C.
However, the present inventors have studied on the processes as described in JP-A-6-227827 and JP-A-8-75901, and as a result, they have found that an adequate durability to ultraviolet light can not always be obtained. Namely, if the porous quartz glass body is treated in an atmosphere containing a fluorine compound at a high temperature of from 800 to 1,250° C., the above-mentioned ≡Si—Si≡ defect will be formed. Not only this ≡Si—Si≡ defect will form the E′ center due to irradiation with ultraviolet lights as mentioned above, but also it has absorption bands at 245 nm and 163 nm, such being problematic.
Further, the ≡Si—Si≡ defect will form ≡Si—H as shown in the following formula (6), even if hydrogen-containing treatment is carried out, and the ≡Si—H will form the E′ center due to irradiation with ultraviolet lights, such being problematic:≡Si—Si≡+H2→≡Si—H+≡Si—H  (6)
On the other hand, in order to improve vacuum ultraviolet lights optical transmittance, JP-A-8-91867 proposes a synthetic quartz glass having an OH group concentration of at most 200 ppm, a chlorine concentration of at most 2 ppm and a ≡Si—Si≡ concentration of at most 1×1015 in number per cm3. JP-A-9-235134 proposes a synthetic quartz glass having an OH group concentration of from 10 to 400 ppm, and the concentrations of the oxygen deficient defect and the oxidation type defect of at most 5×1016 in number per cm3, respectively. JP-A-7-267674 proposes a synthetic quartz glass having an OH group concentration of from 100 to 2,000 ppm and containing a transition metal, an alkali metal and an alkaline earth metal in amounts of at most predetermined concentrations.
With respect to such conventional synthetic quartz glass, improvement in the vacuum ultraviolet lights optical transmittance is attempted by adjusting the OH group concentration to be within a predetermined range. However, a high transmittance can not always be obtained at the vacuum ultraviolet region.
Further, as a process to secure uniformity of the synthetic quartz glass, JP-B-6-27014 proposes a process to adjust the variation ranges of the OH group and chlorine concentrations, by having OH groups and chlorine contained in the synthetic quartz glass. However, chlorine is present in the synthetic quartz glass in the form of ≡Si—Cl, and the bond of ≡Si—Cl has a bonding energy of as weak as from 7 to 8 eV, and it easily undergoes cleavage as shown in the following formula due to irradiation with ultraviolet lights, and the E′ center will be formed also:≡Si—Cl+hν→≡Si. (E′ center)+Cl.
Accordingly, although a synthetic quartz glass having excellent uniformity can be obtained by the above-mentioned process, there is a problem on durability to ultraviolet light.
The present invention provides a synthetic quartz glass which reduces generations of the E′ center and fluorescence emission, and which is excellent in durability to ultraviolet light.
The present invention further provides a synthetic quartz glass which is excellent in vacuum ultraviolet lights optical transmittance.
The present invention further provides a synthetic quartz glass which is excellent in uniformity.
The present invention provides a suitable process for producing such synthetic quartz glass.